1. Field of the Invention
The present invention relates to a method and apparatus for making a sand core with an improved production rate.
2. Description of the Prior Art
Cores and molds used in metal casting consist of a mass of refractory aggregate bound together to form a shape used as a pattern for molten metal during the casting process. The aggregate is typically coated with a binding material and then formed into a shape using a pattern. The binding material is typically hardened to hold the aggregate in the desired shape so the core or mold can be removed from the pattern. The core or mold is then used in giving shape to molten metal so that the metal takes the shape of the original pattern when the metal cools. In common usage, the mold forms the outer surface of the casting and the cores are used to form interior passages in the casting.
One of the most successful current methods for manufacturing cores uses a reactive chemical binder to coat a refractive aggregate such as silica sand. The binder coated sand is blown with air from a sand magazine into a core box having a cavity with a surface of the desired pattern to be used to form the core. The core box also includes vents, which are small openings extending through the core box into the cavity allowing air but not sand to pass through the cavity. Thus the air used to blow the sand into the pattern can escape the cavity while the sand is retained and fills the cavity pattern of the core box. The binder on adjacent sand grains must then be solidified at the contact points between sand grains to ensure that the sand holds the shape of the pattern once the sand core is removed from the core box. The solidification of the binder is often accomplished by passing reactive gas through the sand that reacts with the binder or catalyzes a hardening reaction. Typical examples are amine vapor used to harden phenolic urethane binders and sulfur dioxide gas used to harden acrylic/epoxy binders. Once the reaction has taken place, the reactive gas is usually purged from the core with air. Another type of binder is disclosed in U.S. Pat. No. 5,582,231 to Siak et al. where the hardening of the binder occurs by removal of moisture from the binder.
Typically the core box is divided into two sections which can be opened to remove the core after it has hardened to take the shape of the pattern in the internal cavity of the core box. The division of the core box can be along the horizontal axis where the upper part of the core box is called the cope and the lower part of the core box is called the drag. The division of the core box on the vertical axis results in a left part and a right part of the core box. It is usual for core boxes to have ejection pins along portions of the cavity surface to assist in removing the hardened cores from the core box when the core box parts are separated. These pins are metal rods, of which the ends are flush with the pattern surface of the core box cavity when the core box is closed and the sand is being blown into the box. When the box is opened the pins push against the surface of the core to remove it from the pattern. The pins can be spring-loaded, mechanically forced, or otherwise constructed by suitable means in the art to eject the core. Depending on the shape of the pattern, the ejection pins may be required to exert significant force on the surface of the core. In the drag the pins also support the weight of the core to lift it out of the core box so it can readily be removed from the core box.
The standard procedure for the introducing gas or air into the core box is to use a gassing manifold on the top of the box and an exhaust manifold on the bottom of the box. The gas and/or air passes from the top of the box where it is usually introduced through the blow holes through which the sand is blown into the core box or through vents in the upper surface of the core box. This is an efficient way to introduce reactive gas and purge air in core boxes using these binder systems. A noxious gas such as amine vapor and purge air containing traces of amines pass from the top of the core box, through the core contained within the cavity of the core box, and into the exhaust manifold where it can be collected and directed to a scrubber to remove the noxious gas from the air.
U.S. Pat. No. 5,582,231 to Siak et al. discloses the use of standard core blowing equipment and air to dry the sand core. Traditional core machines are those with purge air flow from the top of the core box to the bottom as described above and as shown in ASM Handbook® (Formerly Ninth Edition, Metals Handbook) Volume 15, “Casting” (1988). However, in the binding system which uses air to remove moisture from the binder to cause hardening (e.g. U.S. Pat. No. 5,582,231), this top to bottom air flow results in an inefficient core making process. The dry air introduced at the top of the core box will become saturated with moisture as it travels down through the hydrated sand in the core. Thus the lower part of the core will be the last part to be dried and hardened because the moisture is pushed downward. In practice this means that a large amount of the total moisture in the core must be removed before the bottom core surface is strong enough to support the force of the ejection pins without breaking and ruining the core when the core box is opened to remove the core. The rate at which cores can be made and removed from the core box, referred to as cycle time, is very important in determining the cost of a core making process. Long cycle times require more capital expense in more core boxes and core machines to produce a given number of cores in a given period of time.